THE PLANT-CELL. 29 



solution on boiling ; of this there are two varieties, dextrose or grape- 

 sugar, which rotates the plane of polarisation to the right, and l&vulose, 

 which rotates it to the left. 



Of the colouring-matters of plants some, such as the green (chloro- 

 phyll) and the yellow, are deposited in corpuscles of a protoplasmic 

 nature, whereas the others, especially the red and blue, are dissolved in 

 the cell-sap. It is to the presence of colouring-matters in solution in 

 the cell-sap that the colours of petals etc. are principally due. 



It has been found that the organic acid which is most commonly 

 present in solution in the cell-sap is malic acid : tartaric and citric acids 

 frequently occur. 



Solid bodies are also commonly present in the vacuole ; 

 for instance, starch-grains, aleurone-grains (in seeds), and 

 crystals of calcium carbonate or oxalate. Calcium oxalate 

 frequently occurs in the form of a bundle of acicular crystals 

 which have been termed raphides. 



BIBLIOGRAPHY. 

 The Cell-wall 



Schmitz ; Ueb. Bildung und Wachsthum der pflanzlichen Zellmem- 



bran, Sitzber. d. niederrh. Ges. fiir Natur- und Heilkunde zu 



Bonn, 1880. 

 Strasburger; Ueb. den Bau und das Wachsthum der Zellhaute, Jena, 



1882. 



Naegeli und Schwendener ; Das Mikroskop, 2nd ed., 1877. 

 Dippel; Das Mikroskop, 1869; also, Die neuere Theorie iib. die 



feinere Structur der Zellhulle, Frankfurt, 1878. 

 Frank ; Ueb. die anatomische Bedeutung und die Entstehung der 



vegetabilischen Schleime, Jahrb. f. wiss. Bot. V, 1867. 

 Kirchner ; Ueb. Pflanzenschleim, Diss. Inaug., Gottingen, 1874. 

 von Hohnel: Ueb. die Cuticula, Oesterr. Bot. Zeitschr. 1878; also, 



Ueb. Kork, Sitzber. d. k. Akad. in Wien, LXXVI, 1877. 

 Frmy ; Recherches chimiques sur la composition des cellules ve'ge'- 



tales, Ann. d. Sci. nat, sdr. 4, t. xn, 1859; ibid. s<r. 6, t. xm. 



1882. 

 Sachs ; Die Porositat des Holzes, Arb. d. bot. Inst. in Wiirzburg, n, 



p. 292, 1879. 

 Erdmann ; Ueb. die Constitution des Tannenholzes, Ann. d. Chemie 



und Pharmacie, Supplement-Band V, 1867. 

 Bevan and Cross ; Contributions to the Chemistry of Bast-fibres, 



Manchester, 1880 : they give also an account of the composition 



of the ash. 



